In the retorting of oil shale for the production of shale oil, nearly all processes developed to date involve passing crushed shale through a gas heated eduction zone maintained at temperatures between about 800.degree. and 1000.degree. F to effect pyrolitic eduction of oil therefrom, leaving educted shale particles still containing about 3-10 weight-percent of coke. Commonly, this remaining coke is burned to generate, either directly or indirectly, the hot gases needed for the eduction zone. Temperatures in the burning zone may range from about 1200.degree. to 2000.degree. F. In other instances, as illustrated in U.S. Pat. No. 3,577,338, the carbon of the educted shale particles is subjected to gasification with superheated steam to generate hydrogen-rich fuel gases. Gasification temperatures are usually in the same range as those encountered in coke combustion zones. In either case, when the educted shale reaches temperatures in excess of about 1200.degree. F, the partial pressure of CO.sub.2 in the gases flowing through the gasification or combustion zone is normally insufficient to suppress completely the decomposition of mineral carbonates to oxides, as e.g.: EQU CaCO.sub.3 .revreaction. CaO + CO.sub.2 ( 1)
The end result is that the spent decarbonized shale contains substantial quantities of relatively water-soluble alkaline metal oxides such as calcium oxide.
With the advent of possible large scale commercial development of oil shale retorting, an ecologically significant problem arises as to safe methods for disposing of the huge volumes of this alkaline spent shale. The only presently conceived methods deemed to be feasible involve either dumping the material in open dumps and eventually revegetating the surface thereof, or returning it to exhausted caverns from which the fresh shale was mined. It either event there is a conceivable danger that for some months after disposal of the spent shale, damage to vegetation and/or aquatic animal life could occur. Aqueous leachings from such spent shale commonly display a pH of around 11-12, and such alkalinity can persist in many volumes of leachate per volume of spent shale. Most plant life, as well as aquatic animal life, are damaged by waters having a pH above about 8.5. Consequently, it is conceivable that for some months after disposal of spent shale, leachings from rain or melting snow could produce ecologically damaging downstream runoff.
The foregoing may appear surprising in view of the common knowledge that calcium oxide in the form for example of quick lime is fairly rapidly converted to calcium carbonate by atmospheric CO.sub.2. However, it appears that the calcium oxide and/or other water-soluble alkaline oxides present in spent shale are in crystalline form which is kinetically very resistant to this recarbonation reaction. Consequently, alkaline runoffs may occur for some time before atmospheric CO.sub.2 can bring about a sufficient recarbonation of the spent shale to avoid this hazard and to form a vegetatable soil.
In view of the foregoing, it is considered a prudent precaution to devise some economical means for reducing the alkalinity of the spent shale by recarbonation before ultimate disposal thereof. On its face, this would seem to be a simple matter in view of the well-known thermodynamics of equation (1) above. As is well known, this reaction will proceed to completion to the left at any temperature below 900.degree. C in the presence of one atmosphere of CO.sub.2. The equilibrium dissociation pressure of CO.sub.2 in contact with the calcium carbonate at 700.degree. C is only 25 mm. It would hence appear to be a simple matter to merely contact the spent shale with waste CO.sub.2 from the retorting operation to effect recarbonation. Quite unexpectedly, this has not been found to be the case; as noted above, the calcium oxide and other water-soluble alkaline oxides are apparently in a crystal form which is remarkably resistant to such recarbonation. Repeated attempts to recarbonate samples of spent shale at economically low partial pressures of CO.sub.2, and within reasonable time periods, have failed.
Undoubtedly recarbonation could be effected with holding times of several days or more in contact with at least one atmosphere of CO.sub.2, but in view of the huge volumes of spent shale to be handled, the investment in tankage would be prohibitive, particularly at super-atmospheric pressures. The prime objective of this invention is to effect adequate recarbonation in a period of time ranging between about 10 minutes and two hours at atmospheric pressure. In many instances this would permit recarbonation during conveyance of the spent shale from the retorting zone to the storage area in a shrouded conveyor through which a stream of CO.sub.2 is passed. In other instances, small holding tanks or towers could be utilized through which CO.sub.2 is passed in contact with a fixed or moving bed of the spent shale.
The present invention is based upon my discovery that the foregoing objectives can be achieved by wetting the spent shale with water containing a small proportion of a dissolved carbonate or bicarbonate salt, and contacting the same with CO.sub.2 at moderate partial pressures of between about 0.5 and 15 psi for times within the aforementioned range. A surprising aspect of the invention is that distilled water, or other waters free of carbonate and bicarbonate salts, are substantially ineffective. This is surprising because, inasmuch as the spent shale still contains substantial quantities of calcium carbonate, CO.sub.2 dissolved in distilled water would be expected to react with the calcium carbonate to yield the relatively soluble salt, calcium bicarbonate. It would appear that both the calcium oxide (which should form calcium hydroxide with water) and the calcium carbonate present in spent shale are quite resistant to reaction with dissolved carbon dioxide, unless the water contains in solution a small proportion of a carbonate and/or bicarbonate salt.
It should be noted that a saturated solution of calcium carbonate in pure water at 25.degree. C has a pH of about 9.4, which is above the safe alkalinity level for most plant and aquatic animal life. However, calcium carbonate is soluble only to the extent of about 0.0015 gms per 100 ml of water at 25.degree. C, whereas calcium oxide is about sixty-six times more soluble. By virtue of the very low solubility of calcium carbonate, only very minor proportions of dissolved CO.sub.2 or bicarbonate salts are sufficient to buffer the pH thereof down to safe levels below about 8.5, generally below 8. Sufficient CO.sub.2 is generally present in rainwaters to produce an adequate buffering proportion of the much more soluble salt, calcium bicarbonate. Even if such were not the case however, high pH leachings of calcium carbonate would very rapidly pick up sufficient atmospheric CO.sub.2 to reduce the pH to safe levels, or would rapidly reach ground waters containing dissolved CO.sub.2 and bicarbonate salts. The high solubility of calcium hydroxide however, as well as its higher alkalinity, render these neutralizing and/or buffering mechanisms much slower in taking effect. Hence the desirability for converting the more soluble, more alkaline oxides to the less soluble, less alkaline carbonates.